Let’s talk about why steel rusts (corrodes)
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Let’s talk about why steel rusts (corrodes). For steel, corrosion is commonly referred to as “rusting”. But why does it rust? Further explanation is still needed. The first understanding of the causes of steel corrosion can be explained from the perspective of energy and energy levels (potential energy). All substances in nature have a natural tendency to transition from higher energy states to lower energy states (just as water flows from high places to low places); this is a law of nature. Corrosion can be understood as the process in which a material, in an environment, undergoes a transition from a higher energy state to a lower energy state, and from an unstable state (metastable state) to a stable state, through physicochemical and electrochemical mechanisms. The second aspect is to explain and understand the corrosion process from the perspective of oxidation-reduction. Corrosion is an oxidation process: iron turns into iron oxide (rusting), plastics, rubber, and similar materials are oxidized (aging), and even the decay of organisms and human aging (such as the effect of free radicals) are all related to oxidation processes. To vividly understand and explain the corrosion process of steel and the importance of corrosion control, iron ore (iron oxide in its stable state in nature) is transformed into steel (a metastable state) through smelting (which increases the energy level and consumes energy). Steel can be used as a structural material to meet mechanical requirements and the desired performance characteristics. However, in normal environments, metastable iron and steel are constantly transforming into lower-energy, stable states, eventually returning to their ore state (iron oxide, rust). Without protection or with inadequate protection, the lifespan of steel is short; however, by implementing corrosion control (adopting effective protective measures), its time in a stable state can be extended, allowing it to meet the expected service life requirements.
The main reason steel rusts is because it undergoes an oxidation reaction. When the surface of steel comes into contact with water and oxygen, iron atoms react with the oxygen in the air to form iron oxide, which is what we commonly refer to as \"rust\". This process can be viewed as iron transitioning from a high-energy state (metallic iron) to a lower-energy, more stable state (iron oxide). This reaction is not just a chemical reaction; it also involves electrochemical processes. On the surface of steel, there are minor battery effects: iron acts as the anode and is oxidized (losing electrons), while the surrounding environment (such as oxygen in water) acts as the cathode, accepting these electrons and thereby facilitating the oxidation of iron. This electrochemical oxidation reaction causes rust to form on the surface of steel. Humidity, temperature, and other factors in the environment such as salt can all accelerate this corrosion process. Therefore, steel is particularly prone to rusting in humid or salty environments. To prevent or slow down the rusting of steel, methods such as coating protection and cathodic protection are commonly used to enhance its corrosion resistance and service life. .